1. The Field of the Invention
The present invention relates generally to a superconducting wire and methods for manufacturing such a wire. More particularly, the present invention relates to the formation of a superconducting wire using a ceramic material, but without the necessity of sintering the ceramic material.
2. The Background of the Invention
It is now generally recognized that superconducting materials hold a great deal of potential for the advancement of technology. For example, in remarks delivered on July 28, 1987 to the Federal Conference on Commercial Application of Superconductivity, the President of the United States stated that "to most of us laymen, superconductivity was a completely new term, but it wasn't long before we learned the great promise it held out to alter our world for the better--a quantum leap in energy efficiency that would bring with it a host of benefits, not least among them a reduced dependence on foreign oil, a cleaner environment, and a stronger national economy."
The phenomenon of superconductivity has been known since the early part of the twentieth century. Superconductivity is generally defined as a condition which exists when a material meets two tests. These tests include (1) zero dc electrical resistance, and (2) the so-called Meissner effect which comprises the expulsion of a magnetic field much as when one magnet repels another When these two conditions are met, materials are classified as superconducting and exhibit some unique properties, most notably zero resistance. It has generally been observed, however, that superconductivity occurs only at very low temperatures. The onset of superconductivity was typically not observed until temperatures dropped to the range of approximately 20.degree. K. or less.
It is apparent that a material having zero resistance has many important potential applications. For example, there is the potential of providing superconducting electrical transmission lines which would be able to carry electricity without significant loss of electrical power. This would result in the savings of billions of dollars in transmission costs and would allow the development of energy generating facilities, such as nuclear power plants, which may be located great distances from urban centers.
It has also been pointed out that superconductivity may allow motors to be produced which are one-tenth of normal size. Those knowledgeable in the art have also speculated that superconductive materials would allow the production of high speed trains levitated by magnets, as well as the production of computers which would be smaller and faster than those presently known. It has also been predicted that new superconductive data transmission lines could be constructed that would carry one trillion bits of information per second, which is approximately 100 times faster than the fiber optic cables which carry many data transmissions and telephone calls at the present time.
The use of superconductivity in these types of devices is not entirely speculative. For example, it has now been reported that the Argonne National Laboratory constructed a liquid nitrogen cooled superconducting electrical motor during late 1987. In addition, certain types of superconducting materials have been known and employed for some time. Niobium superconducting products have been marketed and have been used in various contexts, such as superconducting magnets and in certain types of medical equipment.
One of the primary limitations on the development of superconducting products, such as motors, transmission lines, and the like, has been the fact that most materials are superconducting only at extremely low temperatures. In order to produce these extremely low temperatures it is necessary to employ very expensive liquid helium as a refrigerant. Clearly, the extremely low temperatures at which known materials become superconductive severely limit their usefulness in practical everyday applications.
Recently, however, it has been observed that certain types of materials, particularly specialized ceramics, exhibit superconductive properties at temperatures significantly above those of traditional superconducting materials. For example, a ceramic of lanthanum-copper-oxide doped with barium has been found to be superconducting above liquid helium temperatures. Research demonstrated that this type of ceramic material could be superconducting above 20.degree. K. and even in the temperature range of liquid nitrogen (above 77.degree. K.). Indeed, these types of superconducting materials have recently been referred to as "90.degree. K. superconductors" because of the fact that superconductivity has been observed in the 90.degree. K. range.
These types of materials are often referred to in the art as "high temperature superconducting materials." That is, they exhibit superconducting properties at temperatures obtainable using liquid nitrogen. These superconductors are important because the production of liquid nitrogen is relatively inexpensive and its use is relatively simple.
In early 1986, a compound having the general chemical formula La.sub.2-x Ba.sub.x CuO.sub.4 was found to exhibit onset of superconductivity near 30.degree. K.. Subsequently, findings were reported with a compound comprised of yttrium, barium, copper and oxygen (YBa.sub.2 Cu.sub.3 O.sub.7) exhibiting superconductivity in the 80.degree. K. to 93.degree. K. range, temperatures well within the range obtainable using liquid nitrogen.
Due to the progressive increase in temperature at which superconductivity is found to exist, it has been suggested that superconductivity may ultimately be found at or near room temperature. Such a development would clear the way for practical application of superconductors in many contexts not yet explored.
Superconducting materials of the type referred to above generally exist as ceramics. This fact has been another limitation on the extensive use of these materials. Ceramics are not easily moldable and usable in forms generally used in electrical equipment and ceramics cannot generally be treated in the same manner as metal wires and the like. Thus, one of the difficulties and limitations in the application of ceramic superconductive materials has been the inability to place those materials in a usable form.
An additional problem encountered in ceramic superconductors is the general necessity of sintering the ceramic material before it is used. In order to form a coherent ceramic superconducting material it is the general process to sinter the material before use. Sintering is generally defined as a process whereby a coherent mass of material is obtained by heating the material, causing interconnections between adjacent grains, but without melting the material.
The problem with sintering is, however, that impurities within the ceramic are permanently incorporated within the sintered ceramic. For example, impurity phases which are not superconducting may be melted during the sintering process. Following sintering the impurity phase is often found to coat the grains of the superconducting ceramic. This coating interrupts the structure of the superconducting material. Furthermore, it prevents the grain-to-grain contact that is critical in producing high critical current density superconducting material.
Thus, it is found that upon sintering, ceramic materials may lose some superconductive performance, and may have reduced critical current densities at liquid nitrogen temperatures.
Accordingly, it would be a substantial advancement in the art to provide a very pure superconducting material which was free of surface impurities. It would also be an advancement in the art to provide methods for manufacturing a superconducting ceramic which included means for removing surface impurities. It would be a related advancement in the art to provide a superconducting wire made of a ceramic which did not require sintering during the initial stages of formation. It would be another advancement in the art to provide a superconducting wire that could be used in much the same manner as traditional metal wires are used. It would also be an advancement in the art to provide a superconducting wire and methods for its production which preserve surface contact between the grains of the ceramic superconducting material.
Such methods and apparatus are disclosed and claimed herein.